This invention relates to a radio communication system for transmitting and receiving data, more particularly relates to a system capable of transmitting message data to a plurality of terminal apparatuses after a master apparatus converts the data into optical signals and radio signals.
A radio paging system or the like is proposed in general and concrete as an example of a radio communication system. The radio paging system can be defined as a personal call system, a so called "beeper," "radio pager," or "a pocket bell" as called by the Japanese, which a specified person having left one's seat (recited as "the paged" hereinafter in this specification) is called and receives messages by a wireless apparatus within a defined area such as a firm, hospital, factory, and so on. Such systems comprise transmitters being arranged on inner surfaces of a building such as ceilings, walls or the like, and receivers being put in the pocket of the paged, and carried with them. The signals transmitted from the transmitters include a signal into which the specified messages are converted, and which is received by receivers, so that the communication of messages is performed.
FIG. 1 shows an example of a conventional radio paging system for performing message communication by radio signals. The system comprises calling apparatuses 1a and 1b which are arranged in different firms A and B respectively, transmitters 2a and 2b which convert message signals supplied from the apparatuses 1a and 1b into radio signals and output the signals, and receivers 3a, 4a, 3b and 4b which are carried by some persons to be paged, respectively. There is not always only one set of transmitters 2a and 2b provided in the firm A or B, but also a plurality of transmitters so as to enlarge the service area. Furthermore, there are not only two receivers 3a and 3b or 4a and 4b provided in each firm A or B, respectively, but also a plurality of receivers in each firm, if necessary.
The above-mentioned radio paging systems for transmitting and receiving the messages by means of radio waves, are divided into two classes, that is, a minute power type of the 400 MHz band or the like, and a low power type of 10 mW or the like.
The advantages of the minute power type system reside in that (a) the constitution of the whole is simple, (b) it is not necessary for users to obtain licenses in some countries, (c) users are able to freely select signal frequencies and transmission bands, and (d) there is little interference where signals affect other electric devices being provided peripherally therewith, though the signals have a short distance for transmission, while the low power type systems have the advantage that transmission distance of signals is longer than minute power type systems.
FIG. 2 shows another example of a conventional radio paging system for transmitting and receiving messages by optical signals. The system comprises a calling apparatus 5a or 5b, into optical signals and transmitting the signals through the air, and optical receivers 7a and 8a or 7b and 8b which are carried by those to be paged.
The conventional systems of light wave signal type have some advantages that (a) as users freely select frequencies and modulation systems of light wave signals converted from the message signals, it is possible to transmit the signals by high-speed and wideband transmission, (b) since partition members such as walls, lockers and so on can prevent leakage of the optical signals from each firm, this type has superior confidentiality, and (c) it is unnecessary for users to obtain licenses in some countries.
The above-mentioned conventional radio paging system, however, has many problems some of which will be described hereinafter.
At first, the minute power paging systems have the problems that (a) as the radio wave signals reach only short distances, it is difficult to provide a service area sufficiently broad for practical use, (b) this type is sensitive to electromagnetic interference (EMI) caused by deterioration of the radio wave environment.
On the other hand, the low power type paging systems also have some problems that the occurrence of a fading in higher frequency bands causes the wave signals to deteriorate, (b) while the wave signals reach long distances, it is impossible to sufficiently ensure confidentiality of communication, (c) as the assigned channels are not sufficiently provided in many countries, users have time limits imposed for use of the systems and have to wait a long time to use them, (d) since users are required to denote the specifications in detail for asking for a license, the selling cost increases, and (e) as the Radio Regulatory Act exercises strict control over the use of radio waves in some countries, paging systems cannot be freely designed and utilized.
Furthermore, the conventional systems of the optical signal type have the problems that (a) it is impossible for the receiver, for instance the receivers 8a and 8b as shown in FIG. 2, which are carried by the person being paged in the shade to receive the message signals even if he is close to the optical transmitter, and (b) the optical signals reach only a short distance in comparison with the low power type paging systems mentioned above.